The subject invention is directed to the art of molding parts and, more particularly, to a method and apparatus for molding parts from two or more materials in a two shot injection molding process.
Although the description of the invention below is facilitated in connection with the molding of a specialized nut used in automotive applications, the subject invention finds utility and is applicable in a wide range of part molding processes.
In order to meet the demands of certain specialized applications, it is often desirable to manufacture parts from two or more materials having different properties. As an example, it may be necessary to form a portion of the part from a first substantially rigid material such as nylon or polypropylene and to form a second portion of the part from a softer material such as, for example, a T.P.E. material. The nylon portion of the part is typically designed to withstand certain loading and stress conditions that the part is expected to experience during use. The softer area of the part formed from the T.P.E. or other similar material is typically designed to provide a seal between the part and another associated structure. Another reason to provide a softer area in the part is to produce a damping engagement surface that is able to withstand and dissipate vibrational energy between the part and one or more associated engagement elements. The softer material portion of the part is designed to dissipate the vibrational energy that might otherwise destroy or dislodge the part from the associated engagement member or cause damage to the part.
One traditional way to manufacture a composite part of the type described above from two or more materials is to use a technique known as two-shot injection molding. Typical two-shot molding apparatus include a set of die members that cooperatively form a closed cavity. In a first position, the die members form a first cavity that defines a first portion of the part. The cavity is filled with a first moldable material during a first injection shot. Thereafter, at least one of the die members forming the first cavity in the injection molding apparatus is moved linearly from a first position closed on the part to a second position spaced from the part. The separation between the part and the translated die member creates a second cavity adapted to receive a second moldable material during a second injection shot. Multiple linear mold motions could be used with multiple materials to create composite parts.
One limitation of this type of molding process is that the second cavity created by the relative linear motion between the respective die members is necessarily uniform in cross-section along the axis of the linear motion between the die cavities. That is, the second cavity created by the gap between the movable die member and the part held in place by the non-moving die members necessarily takes on the profile of an extruded part.
Not all parts, however, are designed to resemble an extruded shape in the volume occupied by the second molding material. This poses a serious limitation on the use of two-shot injection molding apparatus that linearly translate one or more die members between shots.
Another form of two-shot injection molding includes a tool or die member that is adapted to open the mold and rotate from a first position to a second position between injection shots. In that type of apparatus, one or more of the mold die members are provided with secondary cavities that are used overmold the secondary material onto the primary material previously formed in the first injection shot and held in place by the non-moving die members. Processes that open the mold between periods of filling the mold with different materials have been fairly successful in producing a reasonable range of parts.
One important constraint, however, in all two-shot injection molding processes is the time delay between injection shots. The pause between mold fill periods must be as short as possible to ensure good bonding between the first and second molding materials. Overall, the longer the time delay between injection shots, the more likely it is that the resultant part will exhibit poor adhesion properties at the interface between the first and second moldable materials. Preferably, the second material should be introduced into the second molding cavity substantially immediately after the first material fills the first cavity so that the first and second materials have an opportunity to co-diffuse at their respective interface surfaces. Further, the second material should be introduced into the mold before the first material has had an opportunity to substantially cool and shrink.
In two-shot injection molding apparatus that are constrained to open the mold cavity before initiating relative movement between one or more die members to create the second cavity, the time required to execute the mechanical motion of the tool can prohibit the co-diffusion action. Most often, without the use of artificial heater elements or the like in the tool, the first material usually cools excessively in the first mold cavity during manipulation of the respective die members. The result is an inadequate bonding between the first and second materials compromising the integrity of the part.
It has, therefore, been considered desirable to develop a new and improved method and apparatus for molding composite parts from diverse constituent materials into a unitary integrated form. It is further desirable to provide a molding method and apparatus capable of producing parts that have surfaces and shapes that are more complicated than parts having simple extrusion-type cross-sections.